The invention lies in the area of the fabrication of silicon-on-insulator substrates known to the man skilled in the art under the abbreviation SOI.
A SOI substrate comprises an oxide layer buried between a support substrate in silicon and a thin layer of silicon called the <<active layer>> since it is on or in this layer that components are to be fabricated notably for electronics, optics and/or opto-electronics.
A SOI substrate is generally obtained by a layer transfer process using the following main steps:
forming an oxide layer on a donor substrate and/or receiver substrate, these two substrates being in silicon;
forming an area of embrittlement in the donor substrate, the area of embrittlement delimiting said above-mentioned active layer to be transferred;
bonding together the donor and receiver substrates, so that the oxide layer lies at their bonding interface;
splitting the donor substrate along the area of embrittlement and transferring the active silicon layer onto the receiver substrate.
One example of said transfer process is the SMARTCUT® process of which a description can be found in: “Silicon-On-Insulator Technology: Materials to VLSI, 2nd Edition”, by Jean-Pierre Colinge, “Kluwer Academic Publishers”, pages 50-51. In this case, the forming of the area of embrittlement is made by implanting atom and/or ion species.
This weakened region may also consist of a porous region.
It is also possible to use a method consisting of bonding a donor substrate onto the receiver substrate, either one of these substrates being coated with an oxide layer, then of reducing the thickness of the donor substrate via its back face so as only to maintain on the receiver substrate a thin semiconductor layer corresponding to said above-mentioned active layer.
Irrespective of the method in which the SOI substrate is obtained, the structure that is obtained after these transfer and bonding steps is subjected to finishing treatments which may for example comprise: polishing, planarization, cleaning, smoothing and thinning steps.
The purpose of these different finishing steps is notably to improve the roughness of the active silicon layer, to bring it to its desired final thickness and to stabilize the bonding interface.
Amongst these finishing steps for a substrate of SOI type, frequent use is made of a smoothing step conducted by rapid thermal annealing (RTA).
RTA is conducted at high temperature for a short time, in a neutral or reducing atmosphere, for example hydrogen and/or argon. As an example, for a SOI substrate it may be conducted at a temperature in the order of 1200° C., for a time of less than three minutes.
This annealing of RTA type carried out to improve the high frequency roughness of the active silicon layer, is also known to have an encapsulating effect of the edge of the substrate, thereby protecting the buried oxide. In this respect, reference may be made to applicant's patent FR 2 852 143.
RTA is often coupled with one or more sacrificial oxidation steps.
A sacrificial oxidation step consists of an oxidation step followed by a de-oxidation step of the previously formed oxide.
The oxidation step is generally performed using thermal treatment, by wet or dry process. The result is the formation of an oxide layer on the free surface of the active silicon layer of the SOI substrate.
The de-oxidation step is generally performed by immersing the oxidized SOI substrate into an acid bath, to remove the previously formed surface oxide and thereby to bring the active silicon layer to the desired final thickness, by reducing its thickness.
From document WO 2006/070220 a treatment method is known for a substrate e.g. of SOI type whose finishing steps comprise at least one cycle of a rapid thermal annealing RTA/sacrificial oxidation sequence.
However, the chief objective of the method used in this document is to reduce the density of the holes opening into the surface of the active layer of said substrate, since these holes amount to major defects. The solution put forward in this document consists solely of increasing the thickness of the transferred layer before subjecting it to finishing steps. This document does not approach the problem of particle contamination of the substrate surface.
Yet, it would also be desirable to limit defectiveness related to particle contamination which deteriorates the quality of the final SOI substrate obtained.
This particle contamination is related to a phenomenon known to those skilled in the art as “flaking”.
This defect typically results from delamination of the edge of the surface of the SOI substrate during cleaning and/or chemical etching processes. It generates flakes with a size in the order of a few tenths of a micrometer which, once re-deposited on the surface of the SOI substrate, can practically no longer be eliminated.
This flake contamination is even more sensitive when RTA is conducted in a cold wall oven, and in some cases it may reach a rejection level with respect to surpassing low-defect requirements for a substrate of SOI type intended to be subsequently used for the fabrication of components, notably in the electronics sector. With a cold wall oven, contaminant flakes of less than 0.1 μm are observed. Thus, there is a need for processes that reduce flake contamination, and these are now provided by the present invention.